Variable focal length lens system including a focus state reference subsystem

ABSTRACT

A focus state reference subsystem comprising a focus state reference object (FSRO) and reference object optics (ROO) is for use in a variable focal length (VFL) lens system comprising a VFL lens, a controller that modulates its optical power, and a camera located along an imaging path including an objective lens and the VFL lens. The ROO transmits image light from the FSRO along a portion of the imaging path through the VFL lens to the camera. Respective FS reference regions (FSRRs) of the FSRO include a contrast pattern fixed at respective focus positions relative to the ROO. A camera image that includes a best-focus image of a particular FSRR defines a best-focus reference state associated with that FSRR, wherein that best-focus reference state comprises a VFL optical power and/or effective focus position of the VFL lens system through the objective lens.

BACKGROUND Technical Field

This disclosure relates to precision metrology using a high speedvariable focal length (VFL) lens (e.g., in a machine vision inspectionsystem), and more particularly to monitoring the focus state of a VFLimaging system and/or optical power of a high speed variable focallength lens in that imaging system.

Description of the Related Art

Precision non-contact metrology systems such as precision machine visioninspection systems (or “vision systems” for short) may be utilized toobtain precise dimensional measurements of objects and to inspectvarious other object characteristics, and may include a computer, acamera and optical system, and a precision stage that moves to allowworkpiece traversal and inspection. One exemplary prior art system isthe QUICK VISION® series of PC-based vision systems and QVPAK® softwareavailable from Mitutoyo America Corporation (MAC), located in Aurora,Ill. The features and operation of the QUICK VISION® series of visionsystems and the QVPAK® software are generally described, for example, inthe QVPAK 3D CNC Vision Measuring Machine User's Guide, publishedJanuary 2003, which is hereby incorporated by reference in its entirety.This type of system uses a microscope-type optical system and moves thestage to provide inspection images of either small or relatively largeworkpieces.

General-purpose precision machine vision inspection systems aregenerally programmable to provide automated video inspection. Suchsystems typically include GUI features and predefined image analysis“video tools” such that operation and programming can be performed by“non-expert” operators. For example, U.S. Pat. No. 6,542,180, which isincorporated herein by reference in its entirety, teaches a visionsystem that uses automated video inspection including the use of variousvideo tools.

Multi-lens variable focal length (VFL) optical systems may be utilizedfor observation and precision measurement of surface heights, and may beincluded in a microscope and/or precision machine vision inspectionsystem, for example as disclosed in U.S. Pat. No. 9,143,674, which ishereby incorporated herein by reference in its entirety. Briefly, a VFLlens is capable of acquiring multiple images at multiple focal lengths,respectively. One type of known VFL lens is a tunable acoustic gradient(“TAG”) lens that creates a lensing effect using sound waves in a fluidmedium. The sound waves may be created by application of an electricalfield at a resonant frequency to a piezoelectric tube surrounding thefluid medium to create a time-varying density and index of refractionprofile in the lens's fluid, which modulates its optical power andthereby the focal length or effective focus position of the opticalsystem. A TAG lens may be used to periodically sweep a range of focallengths at a resonant frequency of up to several hundred kHz, i.e., at ahigh speed. Such a lens may be understood in greater detail by theteachings of the article, “High speed varifocal imaging with a tunableacoustic gradient index of refraction lens” (Optics Letters, Vol. 33,No. 18, Sep. 15, 2008), which is hereby incorporated herein by referencein its entirety. Tunable acoustic gradient index lenses and relatedcontrollable signal generators are available, for example, from TAGOptics, Inc., of Princeton, N.J. The Model TL2.B.xxx series lenses, forexample, are capable of modulation up to approximately 600 kHz.

While such VFL lenses can change effective focus position at a very highrate, variations in conditions such as temperature slightly alter theirresonant characteristics and give rise to changes in optical power andmodulation frequency, which may affect system performance and accuracy.An imaging system that can provide improvements with regard to suchissues would be desirable.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A variable focal length (VFL) lens system is provided including a VFLlens, a VFL lens controller, a camera, an objective lens, an exposuretime controller, and a focus state reference subsystem according toprinciples disclosed herein. In various implementations, the VFL lensmay be a tunable acoustic gradient index of refraction (TAG) lens. TheVFL lens controller controls a drive signal of the VFL lens toperiodically modulate the optical power of the VFL lens over a range ofoptical powers that occur at respective phase timings within theperiodic modulation. The camera is arranged to receive light transmittedalong an imaging optical path through the VFL lens during an imageexposure and provides a corresponding camera image. The objective lensis arranged to input workpiece light arising from a workpiece during aworkpiece image exposure and transmits the workpiece light along theimaging optical path through the VFL lens and to the camera during theworkpiece image exposure, to provide a workpiece image in acorresponding camera image. An effective focus position in front of theobjective lens during a workpiece image exposure corresponds to theoptical power of the VFL lens during that workpiece image exposure. Theexposure time controller is configured to control an image exposuretiming used for a camera image.

As disclosed herein, the variable focal length (VFL) lens system furthercomprises a focus state reference subsystem comprising at least a focusstate (FS) reference object and a reference object optics configuration.The reference object optics configuration is arranged to input referenceobject light arising from the FS reference object during a referenceobject image exposure, and transmit the reference object light along atleast a portion of the imaging optical path to pass through the VFL lensand to the camera during the reference object image exposure, to providea reference object image in a corresponding camera image. The FSreference object comprises a set of focus state (FS) reference regionsthat include a contrast pattern and that have respective known referenceregion image locations in reference object images and that are fixed atdifferent respective reference region focus positions relative to thereference object optics configuration. When the foregoing elements areconfigured according to principles disclosed herein, a camera image thatincludes a best-focus image of a particular FS reference region definesa system focus reference state associated with that particular FSreference region, and that defined system focus reference statecomprises at least one of a particular VFL optical power or a particulareffective focus position associated with that particular FS referenceregion.

In some implementations, the FS reference object comprises a pluralityof planar pattern surfaces fixed at different respective focus distancesrelative to the reference object optics configuration along its opticalaxis, and the set of focus state (FS) reference regions are arranged onthe plurality of planar pattern surfaces.

In some implementations, the FS reference object comprises at least onepattern surface, at least part of which is not perpendicular to anoptical axis of the reference object optics configuration. Differentportions of the at least one pattern surface are fixed at differentrespective focus distances relative to the reference object opticsconfiguration, and the set of focus state (FS) reference regions arearranged on the different portions of the at least one pattern surface.In some such implementations, the at least one pattern surface maycomprise a planar pattern surface that is not normal to the optical axisof the reference object optics configuration, and the set of focus state(FS) reference regions may comprise different portions of a contrastpattern that extends along the planar pattern surface that is not normalto the optical axis.

In some implementations, the reference object optics configurationcomprises a reference object imaging lens having a focal distance Frefalong its optical axis, and the different respective reference regionfocus positions include at least one respective reference region focusdistance located farther than Fref from the reference object imaginglens and at least one respective reference region focus distance locatedcloser than Fref to the reference object imaging lens. In variousimplementations, Fref may be at least 30 millimeters, or at least 40millimeters, or more.

In some implementations, the VFL lens system is configured such that theworkpiece image is located in a first predetermined area of the cameraimage, and the respective known reference region image locations of theset of (FS) reference regions are located in a second predetermined areaof the camera image that is different than the first predetermined area.In some such implementations, a workpiece image and a reference objectimage are exposed simultaneously in the same camera image.

In some implementations, the set of focus state (FS) reference regionsare configured on the FS reference object such that their respectiveknown reference region image locations in reference object images arelocated along one or more edges of the camera image, and not in acentral area of the camera image. In some such implementations, the VFLlens system is configured such that the workpiece image is located inthe central area of the camera image. In some such implementations, aworkpiece image and a reference object image are exposed simultaneouslyin the same camera image.

In some implementations, the VFL lens system comprises a workpiecestrobe light source configured to provide illumination to the workpieceduring the workpiece image exposure, and the exposure time controller isconfigured to control a timing of the workpiece image exposure bycontrolling a strobe timing of the workpiece strobe light source. Insome implementations, the VFL lens system comprises a reference objectstrobe light source configured to provide illumination to the referenceobject during the reference object image exposure, and the exposure timecontroller is configured to control a timing of the reference objectimage exposure by controlling a strobe timing of the reference objectstrobe light source. In some such implementations, the workpiece strobelight source and the reference object strobe light source are the samelight source, the timing of the workpiece image exposure and thereference object image exposure are simultaneous, and a workpiece imageand a reference object image are included in the same camera image. Inother implementations, the workpiece strobe light source and thereference object strobe light source are different light sources, thetiming of the workpiece image exposure and the reference object imageexposure are different, and a workpiece image and a reference objectimage are included in different camera images.

In some implementations, the VFL lens system comprises a referenceregion focus analyzer configured to identify a best-focus image of aparticular FS reference region in the camera image, and identify theparticular effective focus position associated with that particular FSreference region as an effective focus position of the workpiece imagein the same camera image.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram showing various typical components of ageneral-purpose precision machine vision inspection system;

FIG. 2 is a block diagram of a control system portion and a visioncomponents portion of a machine vision inspection system similar to thatof FIG. 1 and including certain features disclosed herein;

FIG. 3 is a schematic diagram of a VFL imaging system that may beadapted to a precision non-contact metrology system such as a machinevision inspection system, including a focus state reference subsystemaccording to principles disclosed herein;

FIG. 4 is a diagram showing an implementation including a firstembodiment of a reference object usable in a focus state referencesubsystem according to principles disclosed herein.

FIGS. 5A, 5B and 5C are diagrams representing three camera images thatinclude images of the reference object of FIG. 4 in three differentfocus states.

FIG. 6 is a chart representing the relationships between various focusstate features or parameters associated with various “best focused”reference regions of the reference object represented in FIGS. 4 and 5,for a VFL imaging system that includes a focus state reference subsystemaccording to principles disclosed herein.

FIG. 7 is a chart representing the relationships between various focusstate features or parameters associated with a periodically modulatedfocus state of a VFL imaging system that includes a focus statereference subsystem according to principles disclosed herein.

FIG. 8 is a diagram showing an implementation including a secondembodiment of a reference object usable in a focus state referencesubsystem according to principles disclosed herein.

FIGS. 9A, 9B and 9C are diagrams representing three camera images thatinclude images of the reference object of FIG. 8 in three differentfocus states.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one exemplary machine vision inspectionsystem 10 usable as an imaging system in accordance with methodsdescribed herein. The machine vision inspection system 10 includes avision measuring machine 12 that is operably connected to exchange dataand control signals with a controlling computer system 14. Thecontrolling computer system 14 is further operably connected to exchangedata and control signals with a monitor or display 16, a printer 18, ajoystick 22, a keyboard 24, and a mouse 26. The monitor or display 16may display a user interface suitable for controlling and/or programmingthe operations of the machine vision inspection system 10. It will beappreciated that in various implementations, a touchscreen tablet or thelike may be substituted for and/or redundantly provide the functions ofany or all of the elements 14, 16, 22, 24 and 26.

Those skilled in the art will appreciate that the controlling computersystem 14 may generally be implemented using any suitable computingsystem or device, including distributed or networked computingenvironments, and the like. Such computing systems or devices mayinclude one or more general-purpose or special-purpose processors (e.g.,non-custom or custom devices) that execute software to perform thefunctions described herein. Software may be stored in memory, such asrandom-access memory (RAM), read-only memory (ROM), flash memory, or thelike, or a combination of such components. Software may also be storedin one or more storage devices, such as optical-based disks, flashmemory devices, or any other type of non-volatile storage medium forstoring data. Software may include one or more program modules thatinclude routines, programs, objects, components, data structures, and soon that perform particular tasks or implement particular abstract datatypes. In distributed computing environments, the functionality of theprogram modules may be combined or distributed across multiple computingsystems or devices and accessed via service calls, either in a wired orwireless configuration.

The vision measuring machine 12 includes a moveable workpiece stage 32and an optical imaging system 34 that may include a zoom lens orinterchangeable objective lenses. The zoom lens or interchangeableobjective lenses generally provide various magnifications for the imagesprovided by the optical imaging system 34. Various implementations ofthe machine vision inspection system 10 are also described in commonlyassigned U.S. Pat. Nos. 7,454,053; 7,324,682; 8,111,905; and 8,111,938,each of which is hereby incorporated herein by reference in itsentirety.

FIG. 2 is a block diagram of a control system portion 120 and a visioncomponents portion 200 of a machine vision inspection system 100 similarto the machine vision inspection system of FIG. 1, including certainfeatures disclosed herein. As will be described in more detail below,the control system portion 120 is utilized to control the visioncomponents portion 200. The vision components portion 200 includes anoptical assembly portion 205, light sources 220, 230, 240, and aworkpiece stage 210 having a central transparent portion 212. Theworkpiece stage 210 is controllably movable along x- and y-axes that liein a plane that is generally parallel to the surface of the stage wherea workpiece 20 may be positioned.

The optical assembly portion 205 includes a camera system 260, aninterchangeable objective lens 250, a variable focal length (VFL) lens270 (e.g., a TAG lens in various exemplary implementations), and a focusstate reference subsystem 286, as disclosed herein in greater detailbelow. In various implementations, the optical assembly portion 205 mayfurther include a turret lens assembly 223 having lenses 226 and 228. Asan alternative to the turret lens assembly, in various implementations afixed or manually interchangeable magnification-altering lens, or a zoomlens configuration, or the like, may be included. In variousimplementations, the interchangeable objective lens 250 may be selectedfrom a set of fixed magnification objective lenses that are included aspart of the variable magnification lens portion (e.g., a set ofobjective lenses corresponding to magnifications such as 0.5×, 1×, 2× or2.5×, 5×, 10×, 20× or 25×, 50×, 100×, etc.)

The optical assembly portion 205 is controllably movable along a z-axisthat is generally orthogonal to the x- and y-axes by using acontrollable motor 294 that drives an actuator to move the opticalassembly portion 205 along the z-axis to change the focus of the imageof the workpiece 20. The controllable motor 294 is connected to aninput/output interface 130 via a signal line 296. As will be describedin more detail below, to change the focus of the image over a smallerrange, or as an alternative to moving the optical assembly portion 205,the VFL (TAG) lens 270 may be controlled via a signal line 234′ by alens control interface 134 to periodically modulate the optical power ofthe VFL lens 270 and thus modulate an effective focus position of theoptical assembly portion 205. The lens control interface 134 may includea VFL lens controller 180 according to various principles disclosedherein, as described in greater detail below. A workpiece 20 may beplaced on the workpiece stage 210. The workpiece stage 210 may becontrolled to move relative to the optical assembly portion 205, suchthat the field of view of the interchangeable objective lens 250 movesbetween locations on the workpiece 20, and/or among a plurality ofworkpieces 20.

One or more of a stage light source 220, a coaxial light source 230, anda surface light source 240 (e.g., a ring light) may emit source light222, 232, and/or 242, respectively, to illuminate the workpiece orworkpieces 20. For example, during an image exposure, the coaxial lightsource 230 may emit source light 232 along a path including a beamsplitter 290 (e.g., a partial mirror). The source light 232 is reflectedor transmitted as workpiece light 255, and the workpiece light used forimaging passes through the interchangeable objective lens 250, theturret lens assembly 223 and the VFL lens 270 and is gathered by thecamera system 260. A workpiece image exposure which includes the imageof the workpiece(s) 20, is captured by the camera system 260, and isoutput on a signal line 262 to the control system portion 120.

In the illustrated implementation, the beam splitter 290 may transmitsome of the source light 232 to be reflected by a mirror 291 toilluminate a reference object included in the focus state referencesubsystem 286, as described in greater detail below. In such animplementation, reference object light 289 may be returned to the mirror291 and the beam splitter 290 and reflected to continue along at least aportion of the imaging optical path to pass through the VFL lens 270 andto the camera system 260 during a reference object image exposure, toprovide a reference object image in a corresponding camera image, asdescribed in greater detail below. However, in other implementations,the focus state reference subsystem 286 may include a separatelycontrolled illumination source (e.g., as described below with referenceto FIG. 3.)

Various light sources (e.g., the light sources 220, 230, 240) may beconnected to a lighting control interface 133 of the control systemportion 120 through associated signal lines (e.g., the busses 221, 231,241, respectively). In various implementations, this may include a lightsource of the focus state reference subsystem 286, as described furtherbelow. The control system portion 120 may control the turret lensassembly 223 to rotate along axis 224 to select a turret lens through asignal line or bus 223′ to alter an image magnification.

As shown in FIG. 2, in various exemplary implementations, the controlsystem portion 120 includes a controller 125, the input/output interface130, a memory 140, a workpiece program generator and executor 170, and apower supply portion 190. Each of these components, as well as theadditional components described below, may be interconnected by one ormore data/control busses and/or application programming interfaces, orby direct connections between the various elements. The input/outputinterface 130 includes an imaging control interface 131, a motioncontrol interface 132, a lighting control interface 133, and the lenscontrol interface 134. The lens control interface 134 may include or beconnected to a VFL lens controller 180 including circuits and/orroutines for controlling various image exposures synchronized with theperiodic focus position modulation provided by the VFL lens 270, andincluding focus state reference subsystem circuits/routines 183according to principles disclosed herein, as described in greater detailbelow with reference to similar or identical elements 380 and 383 shownin FIG. 3. In some implementations, the lens control interface 134 andthe VFL lens controller 180 may be merged and/or indistinguishable.

The lighting control interface 133 may include lighting control elements133 a-133 n, that control, for example, the selection, power, on/offswitch, and strobe pulse timing, if applicable, for the variouscorresponding light sources of the machine vision inspection system 100.In some embodiments, an exposure (strobe) time controller 333 es asshown in FIG. 3 may provide strobe timing signals to one or more of thelighting control elements 133 a-133 n, such that they provide an imageexposure strobe timing that is synchronized with a desired phase time ofthe VFL lens focus position modulation, and as described in greaterdetail below. In some implementations, the exposure (strobe) timecontroller 333 es and one or more of the lighting control elements 133a-133 n may be merged and/or indistinguishable.

The memory 140 may include an image file memory portion 141, anedge-detection memory portion 140 ed, a workpiece program memory portion142 that may include one or more part programs, or the like, and a videotool portion 143. The video tool portion 143 includes video tool portion143 a and other video tool portions (e.g., 143 n) that determine theGUI, image-processing operation, etc., for each of the correspondingvideo tools, and a region of interest (ROI) generator 143 roi thatsupports automatic, semi-automatic, and/or manual operations that definevarious ROIs that are operable in various video tools included in thevideo tool portion 143. Examples of the operations of such video toolsfor locating edge features and performing other workpiece featureinspection operations are described in more detail in certain of thepreviously incorporated references, as well as in U.S. Pat. No.7,627,162, which is hereby incorporated herein by reference in itsentirety.

The video tool portion 143 also includes an autofocus video tool 143 afthat determines the GUI, image-processing operation, etc., for focusheight measurement operations. In various implementations, the autofocusvideo tool 143 af may additionally include a high-speed focus heighttool that may be utilized to measure focus heights with high speed usinghardware described in FIG. 3, as described in more detail in U.S. Pat.No. 9,143,674, which is hereby incorporated herein by reference in itsentirety. In various implementations, the high-speed focus height toolmay be a special mode of the autofocus video tool 143 af that mayotherwise operate according to conventional methods for autofocus videotools, or the operations of the autofocus video tool 143 af may onlyinclude those of the high-speed focus height tool. High-speed autofocusand/or focus position determination for an image region or regions ofinterest may be based on analyzing the image to determine acorresponding quantitative contrast metric for various regions,according to known methods. For example, such methods are disclosed inU.S. Pat. Nos. 8,111,905; 7,570,795; and 7,030,351, which are herebyincorporated herein by reference in their entirety.

In the context of this disclosure, and as is known by one of ordinaryskill in the art, the term “video tool” generally refers to a relativelycomplex set of automatic or programmed operations that a machine visionuser can implement through a relatively simple user interface. Forexample, a video tool may include a complex pre-programmed set ofimage-processing operations and computations that are applied andcustomized in a particular instance by adjusting a few variables orparameters that govern the operations and computations. In addition tothe underlying operations and computations, the video tool comprises theuser interface that allows the user to adjust those parameters for aparticular instance of the video tool. It should be noted that thevisible user interface features are sometimes referred to as the videotool, with the underlying operations being included implicitly.

One or more display devices 136 (e.g., the display 16 of FIG. 1) and oneor more input devices 138 (e.g., the joystick 22, keyboard 24, and mouse26 of FIG. 1) may be connected to the input/output interface 130. Thedisplay devices 136 and input devices 138 may be used to display a userinterface that may include various graphical user interface (GUI)features that are usable to perform inspection operations, and/or tocreate and/or modify part programs, to view the images captured by thecamera system 260, and/or to directly control the vision componentsportion 200.

In various exemplary implementations, when a user utilizes the machinevision inspection system 100 to create a part program for the workpiece20, the user generates part program instructions by operating themachine vision inspection system 100 in a learn mode to provide adesired image-acquisition training sequence. For example, a trainingsequence may comprise positioning a particular workpiece feature of arepresentative workpiece in the field of view (FOV), setting lightlevels, focusing or autofocusing, acquiring an image, and providing aninspection training sequence applied to the image (e.g., using aninstance of one of the video tools on that workpiece feature). The learnmode operates such that the sequence(s) are captured or recorded andconverted to corresponding part program instructions. Theseinstructions, when the part program is executed, will cause the machinevision inspection system to reproduce the trained image acquisition andcause inspection operations to automatically inspect that particularworkpiece feature (that is the corresponding feature in thecorresponding location) on a run mode workpiece, or workpieces, whichmatches the representative workpiece used when creating the partprogram. In some implementations, such techniques may be utilized tocreate a part program instruction for analyzing a reference objectimage, to provide functions and operations described in more detailbelow.

FIG. 3 is a schematic diagram of a VFL lens system 300 (also referred toas imaging system 300) that includes a VFL lens 370 (e.g., a TAG lens)and a focus state reference subsystem 386 configured according toprinciples disclosed herein. The VFL lens system 300 may be adapted to amachine vision system or configured as a standalone system, and may beoperated according to principles disclosed herein. It will beappreciated that certain numbered components 3XX of FIG. 3 maycorrespond to and/or provide similar operations or functions assimilarly numbered components 2XX of FIG. 2, and maybe similarlyunderstood unless otherwise indicated. As will be described in moredetail below, an imaging optical path OPATH comprises various opticalcomponents arranged along a path that conveys workpiece imaging light355 from the workpiece 320 to the camera 360. The imaging light isgenerally conveyed along the direction of their optical axes OA. In theimplementation shown in FIG. 3, all the optical axes OA are aligned.However, this implementation is exemplary only and not limiting. Moregenerally, the imaging optical path OPATH may include mirrors and/orother optical elements, and may take any form that is operational forimaging the workpiece 320 using a camera (e.g., the camera 360)according to known principles. In the illustrated implementation, theimaging optical path OPATH includes the VFL lens 370 (which may beincluded in a 4f imaging configuration) and is utilized at least in partfor imaging a surface of a workpiece 320 during a workpiece imageexposure. As will be described in more detail below, in accordance withprinciples disclosed herein, the focus state reference subsystem 386 maybe utilized to transmit the reference object light along at least aportion of the imaging optical path OPATH to pass through the VFL lens370 to form one or more reference object image exposures, which may beanalyzed for their focus characteristics and compared to thecorresponding stored characteristics associated with system focusreference states (e.g., calibration states) to enable sensing of changesor errors in the expected optical power of the VFL lens 370 and/or theeffective focus position of the VFL lens system 300.

As shown in FIG. 3, the VFL lens system 300 includes a light source 330,an objective lens 350, a tube lens 351, a relay lens 352, a VFL (TAG)lens 370, a relay lens 356, a lens controller 380, a camera 360, aneffective focus position (Z-height) calibration portion 373, a workpiecefocus signal processing portion 375 (optional), and a focus statereference subsystem 386. In various implementations, the variouscomponents may be interconnected by direct connections or one or moredata/control busses (e.g., a system signal and control bus 395) and/orapplication programming interfaces, etc.

In the implementation shown in FIG. 3, the light source 330 may be a“coaxial” or other light source configured to emit the source light 332(e.g., with strobed or continuous illumination) along a path including abeam splitter 390 (e.g., a partially reflecting mirror as part of a beamsplitter) and through the objective lens 350 to a surface of a workpiece320, wherein the objective lens 350 receives the workpiece light 355that is focused at an effective focus position EFP proximate to theworkpiece 320, and outputs the workpiece light 355 to the tube lens 351.The tube lens 351 receives the workpiece light 355 and outputs it to therelay lens 352. In other implementations, analogous light sources mayilluminate the field of view in a non-coaxial manner; for example a ringlight source may illuminate the field of view. In variousimplementations, the objective lens 350 may be an interchangeableobjective lens and the tube lens 351 may be included as part of a turretlens assembly (e.g., similar to the interchangeable objective lens 250and the turret lens assembly 223 of FIG. 2). In various implementations,any of the other lenses referenced herein may be formed from or operatein conjunction with individual lenses, compound lenses, etc.

The relay lens 352 receives the workpiece light 355 and outputs it tothe VFL (TAG) lens 370. The VFL (TAG) lens 370 receives the workpiecelight 355 and outputs it to the relay lens 356. The relay lens 356receives the workpiece light 355 and outputs it to the camera 360. Invarious implementations, the camera 360 captures a camera image duringan image exposure (e.g., during an integration period of the camera 360)also referred to as an image exposure period, and may provide thecorresponding image data to a control system portion. Some camera imagesmay include a workpiece image (e.g., of a region of the workpiece 320)provided during a workpiece image exposure. In some embodiments, theworkpiece image exposure may be limited or controlled by a strobe timingof the light source 330 that falls within an image integration period ofthe camera 360. As described in greater detail below, some camera imagesmay include a reference object image (e.g., of a reference object 388)provided during a reference object image exposure. In some embodiments,the reference object image exposure may be limited or controlled by astrobe timing of the light source 330 or 330 ro that falls within animage integration period of the camera 360. In various implementations,the camera 360 may have a pixel array greater than 1 megapixel (e.g.,1.3 megapixel, with a 1280×1024 pixel array, with 5.3 microns perpixel).

In the example of FIG. 3, the relay lenses 352 and 356 and the VFL (TAG)lens 370 are designated as being included in a 4f optical configuration,while the relay lens 352 and the tube lens 351 are designated as beingincluded in a Keplerian telescope configuration, and the tube lens 351and the objective lens 350 are designated as being included in amicroscope configuration. All of the illustrated configurations will beunderstood to be exemplary only, and not limiting with respect to thepresent disclosure. In various implementations, the illustrated 4 foptical configuration permits placing the VFL (TAG) lens 370 (e.g.,which may be a low numerical aperture (NA) device) at the Fourier planeof the objective lens 350. This configuration may maintain thetelecentricity at the workpiece 320 and may minimize scale change andimage distortion (e.g., including providing constant magnification foreach Z-height of the workpiece 320 and/or effective focus position EFP).The Keplerian telescope configuration (e.g., including the tube lens 351and the relay lens 352) may be included between the microscopeconfiguration and the 4 f optical configuration, and may be configuredto provide a desired size of the projection of the objective lens clearaperture at the location of the VFL (TAG) lens 370, so as to minimizeimage aberrations, etc.

In various implementations, the lens controller 380 may include a drivesignal generator portion 381, a timing clock 381′, workpiece imagingcircuits/routines 382 and focus state reference subsystemcircuits/routines 383. The drive signal generator portion 381 mayoperate (e.g., in conjunction with the timing clock 381′) to provide aperiodic drive signal to the high speed VFL (TAG) lens 370 via a signalline 380′. In various implementations, the VFL lens system (or imagingsystem) 300 may comprise a control system (e.g., the control systemportion 120 of FIG. 2) that is configurable to operate in conjunctionwith the lens controller 380 for coordinated operations.

In various implementations, the lens controller 380 may generallyperform various functions related to imaging a workpiece 320 orreference object 388 in a manner synchronized with a desired phasetiming of the VFL lens 370, as well as controlling, monitoring andadjusting the driving and response of the VFL lens 370. In variousimplementations, the workpiece imaging circuits/routines 382 performstandard workpiece imaging operations for the optical system,synchronized with the phase timing of the VFL lens 370 as known in theart and as described in the incorporated references. As will bedescribed in more detail below, in various implementations the focusstate reference subsystem circuits/routines 383 may perform focus statemonitoring and/or stabilization in accordance with principles disclosedherein. In various implementations, the focus state monitoring and/orstabilization may be performed on an on-demand basis (e.g., in responseto a user selection in a user interface, or when a particular conditionis detected, etc.), or may be performed periodically (once every second,or 10 seconds, or hour, etc.). In some implementations, the lenscontroller 380 may operate such that a reference object image exposurerequired for focus state monitoring does not overlap with a workpieceimage exposure, although any adjustments to the system (e.g., to adjustthe operation of the VFL lens 370) determined during the focus statemonitoring will continue to be applied and utilized during subsequentworkpiece imaging.

The focus state reference subsystem circuits/routines 383 include areference region focus analyzer 384 and adjustment and/or warningcircuits/routines 385. In various implementations, the reference regionfocus analyzer 384 may perform functions such as inputting referenceobject images (e.g., as included in camera images) and calling certainvideo tools (e.g., a known type of autofocus video tool, or multi-regionor multi-point autofocus video tool, or the like) or other focusanalysis routines to determine one or more focus characteristic values(e.g., a quantitative contrast and/or focus metric) for focus statereference regions (FSRRs) in the reference object images used for focusstate monitoring, etc. In various implementations, the adjustment and/orwarning circuits/routines 385 may input the determined focuscharacteristic results/values from reference region focus analyzer 384,and may compare the results/values to corresponding stored calibrationresults/values for corresponding FS reference regions, in order todetermine whether adjustments need to be made. As will be described inmore detail below, in various implementations adjustments may include(but are not limited to) adjusting an amplitude for driving the VFL lens370 (e.g., for adjusting its optical power range and the resultingeffective focus position range), a phase timing adjustment (e.g., foradjusting the phase timing used to provide particular effective focuspositions or Z-heights), a VFL lens temperature adjustment, etc. Invarious implementations, such adjustments may be implemented throughchanges to the control signals of the drive signal generator portion381, timing clock 381′, and/or lens heater/cooler 337, etc., as will bedescribed in more detail below. In various implementations, the focusstate reference subsystem circuits/routines 383 may in some instancesrepeatedly perform operations to iteratively analyze and adjust thesystem until the optical power range of the VFL lens and/or theresulting effective focus position range is at desired levels (e.g.,within a desired tolerance relative to stored calibration or referencelevels).

Drift in the operating characteristics of the VFL lens may arise due tounwanted temperature variations. As shown in FIG. 3, in variousimplementations, the imaging system 300 may optionally include the lensheater/cooler 337 associated with the VFL lens 370. The lensheater/cooler 337 may be configured to input an amount of heat energyinto the VFL lens 370 and/or perform cooling functions to facilitateheating and/or cooling of the VFL lens 370 according to someimplementations and/or operating conditions. In addition, in variousimplementations a VFL lens monitoring signal may be provided by atemperature sensor 336 associated with the VFL lens 370 to monitor anoperating temperature of the VFL lens 370.

As will be described in more detail below, during focus statemonitoring, reference object image exposures may be provided by usingthe camera 360 to capture images of the reference object 388 through thereference object optics configuration 387, which are both included inthe focus state reference subsystem 386. In some implementations, someof the light 332 from the light source 330 may pass through the beamsplitter 390 and may be used for reference object image exposures. Inother implementations, the focus state reference subsystem 386 may alsocomprise a reference object light source 330 ro that provides a light332 ro that is used for reference object image exposures. The referenceobject light source 330 ro may be connected to and controlled by signalsand/or controlled power over the system signal and control bus 395,which may be governed by the exposure time controller 333 es, or thelike, as previously outlined. In any case, reference object light arisesfrom the FS reference object and the reference object opticsconfiguration 387 transmits the reference object light along at least aportion of the imaging optical path OPATH to pass through the VFL lens370 and to the camera 360 during a reference object image exposure, toprovide a reference object image in a corresponding camera image (e.g.,as will be described in more detail below with respect to FIGS. 4, 5, 8and 9). In some implementations, the focus state reference subsystem 386may be configured to transmit the reference object light from a Fourierplane of the microscope configuration of the objective lens 350 and thetube lens 351, if desired. As will be described in more detail belowwith respect to FIGS. 5A-5C, the camera 360 may provide reference objectimages (e.g., such as the exemplary images 500A-500C) exposed duringcorresponding phase timings of the periodic modulation of the VFL lens370 and the resulting effective focus position of the imaging system 300to support focus state monitoring operations. As explained in greaterdetail below, focus characteristic values for members of a set of FSreference regions included in reference object images exposed usingparticular known phase timings (e.g., such as the exemplary images500A-500C) are related to an optical power of the VFL lens 370 and theresulting effective focus position of the imaging system 300 during thecorresponding phase timings.

In various implementations, the FS reference object 388 comprises a setof focus state (FS) reference regions that include a contrast patternand that have respective known reference region image locations inreference object images and that are fixed at a different respectivereference region focus distances or positions relative to the referenceobject optics configuration 387, as explained in greater detail belowwith reference to FIGS. 4, 5, 8 and 9. As a result, a camera image thatincludes a best-focus image of a particular FS reference region definesa system focus reference state associated with that particular FSreference region. That defined system focus reference state comprises atleast one of a particular VFL optical power or a particular effectivefocus position associated with that particular FS reference region, asdescribed further below with reference to FIGS. 6 and 7. In theillustrated implementation, the reference object optics configuration387 comprises a lens 387 a which transmits reference object light 389that arises from the FS reference object 388 toward the beam splitter390. The partially reflecting beam splitter 390 directs the light 389 ofthe reference object image along the imaging optical path OPATH, whichpasses through the VFL lens 370 and emerges as light 389′ which formsthe image of the reference object in the reference object image exposurethat is produced by the camera 360 to support focus state monitoring.

In various implementations, as described in greater detail below, thelens 387 a is generally configured to provide the desired divergence, ornear collimation, for the reference object light 389 that arises fromthe FS reference object 388 and results in the reference object image.It will be appreciated that in other implementations, other locationsand/or configurations of the components of the focus state referencesubsystem 386 may be utilized. For example, in various alternativeimplementations, the lens 387 a of the reference object opticsconfiguration 387 may be a compound lens, or a set of lenses may beused, and/or one or more additional reflectors may be used to reshapethe reference object optical path (e.g., OAref), and/or wavelengthfilters may be included (in combination with using a specific wavelengthrange in the light source 330 ro) to isolate workpiece images andreference object images from one another according to known principles,or the like. In one implementation, a wavelength filter may be shapedand located to filter narrow band reference object light 389 from thatportion of the camera image (e.g., the central portion) that is intendedto receive only the workpiece light 355, and to not filter the narrowband reference object light 389 from that portion of the camera image(e.g., the peripheral portion) that is intended to provide a referenceobject image. In some implementations, wavelength filters (e.g.,dichroic filters) may be added to one or more elements depicted in FIG.3, (e.g., the beam splitter 390). In other implementations, wavelengthfilters may be added as separate elements. In another configuration, thefocus state reference subsystem 386 may be located on the same side ofthe beam splitter 390 as the light source 330, with correspondingtransmissive and/or reflective properties of the beam splitter 390and/or additional reflective surfaces utilized for directing thereference object light 389 along the imaging optical path OPATH.

In various implementations, the light 389 of the reference object imagehas a pattern and beam divergence that is determined by the referenceobject optics configuration 387 of focus state reference subsystem 386.The VFL lens 370 receives the light 389 of the reference object imageand outputs the light 389′ of the reference object image, for which theimage focus location (e.g., at the camera 360) is periodically alteredby the periodic optical power variation associated with the operation ofthe VFL lens 370. It will be appreciated that the point conjugate to thecamera image plane, that is, the focused plane in the vicinity of thereference object 388 is thus also periodically altered or swept due tothe periodic optical power variation associated with the operation ofthe VFL lens 370. When different respective focus state (FS) referenceregions on the reference object 388 are located at different respectivedistances from the reference object optics configuration 387, they willthus be focused in respective images acquired at different respectivetimes during the periodic optical power variation of the VFL lens 370.Thus, a focus state reference subsystem 386 including such a referenceobject 388 may be used to determine whether the VFL lens system 300 isstable in its operation, and/or in a desired (e.g., calibrated) focusstate at various times during the periodic optical power variation ofthe VFL lens 370, and/or to directly determine the effective focusposition of a particular image, in various implementations, as describedin greater detail below.

With respect to the general operations of the VFL lens 370, in variousimplementations as described above, the lens controller 380 may rapidlyadjust or modulate its optical power periodically, to achieve ahigh-speed VFL lens capable of a periodic modulation (i.e., at a VFLlens resonant frequency) of 250 kHz, or 70 kHz, or 30 kHz, or the like.As shown in FIG. 3, by using the periodic modulation of a signal todrive the VFL lens 370, the effective focus position EFP of the imagingsystem 300 (that is, the focus position in front of the objective lens350) may be (rapidly) moved within a range Refp (e.g., an autofocussearch range) bound by an effective focus position EFP1 (or EFPmax)corresponding to a maximum optical power of the VFL lens 370 incombination with the objective lens 350, and an effective focus positionEFP2 (or EFPmin) corresponding to a maximum negative optical power ofthe VFL lens 370 in combination with the objective lens 350. In variousimplementations, EFP1 and EFP2 may approximately correspond to phasetimings of 90 degrees and 270 degrees, as will be described in moredetail below). For purposes of discussion, the middle of the range Refpmay be designated as EFPnom, and may correspond to zero optical power ofthe VFL lens 370 in combination with the nominal optical power of theobjective lens 350. According to this description, EFPnom mayapproximately correspond to the nominal focal length of the objectivelens 350 in some implementations.

In one implementation, the optional workpiece focus signal processingportion 375 (optional) may input data from the camera 360 and mayprovide data or signals that are utilized to determine when an imagedsurface region (e.g., of the workpiece 320) is at an effective focusposition in an image. For example, a group of images acquired by thecamera 360 at different Z-heights (e.g., an image stack), may beanalyzed using a known “maximum contrast” or “best-focus image” analysisto determine if or when an imaged surface region of the workpiece 320 isat corresponding effective focus position in an image. However, moregenerally, any other suitable known image focus detection configurationmay be used. In any case, the workpiece focus signal processing portion375 or the like may input an image or images acquired during theperiodic modulation of the effective focus position (sweeping ofmultiple effective focus positions) of the VFL (TAG) lens 370, anddetermine an image wherein a target feature is best-focused. In someimplementations, the workpiece focus signal processing portion 375 mayfurther determine the known phase timing corresponding to thatbest-focus image and output that “best-focus” phase timing value to theeffective focus position calibration portion 373. The effective focusposition calibration portion 373 may provide Z-height (effective focusposition) calibration data that relates respective Z-heights oreffective focus positions to respective “best-focus” phase timingswithin a period of a standard imaging resonant frequency of the VFL lens370, wherein the calibration data corresponds to operating the VFL lens370 according to a standard imaging drive control configuration orreference state.

Generally speaking, the effective focus position calibration portion 373comprises recorded Z-height (or effective focus position) calibrationdata. As such, its representation in FIG. 3 as a separate element isintended to be a schematic representation only, and not limiting. Invarious implementations, the associated recorded Z-height calibrationdata may be merged with and/or indistinguishable from the lenscontroller 380, the workpiece focus signal processing portion 375, or ahost computer system connected to the system signal and control bus 395,etc.

In various implementations, the exposure (strobe) time controller 333 escontrols an image exposure time of the imaging system 300 (e.g.,relative to a phase timing of the periodically modulated effective focusposition). Specifically, in some implementations, during an imageexposure the exposure (strobe) time controller 333 es (e.g., using theZ-height calibration data available in the effective focus positioncalibration portion 373), may control the light source 330 (and/or 330ro) to strobe at a respective controlled time. For example, the exposure(strobe) time controller 333 es may control the strobe light source tostrobe at a respective phase timing within a period of a standardimaging resonant frequency of the VFL lens 370, so as to acquire animage having a particular effective focus position within the sweeping(periodic modulation) range of the VFL lens 370. In otherimplementations, the exposure time controller 333 es may control a fastelectronic camera shutter of the camera 360 to acquire an image at arespective controlled time and/or its associated effective focusposition. In some implementations, the exposure (strobe) time controller333 es may be merged with or indistinguishable from the camera 360. Itwill be appreciated that the operations of the exposure time controller333 es and other features and elements outlined above may be implementedto govern workpiece image acquisitions, reference object imageacquisitions, or both, in various implementations. As will be describedin more detail below with respect to FIGS. 5A-5C and FIG. 9A-9C incertain specific example implementations, reference object imageexposures may thus be controlled to correspond to specified phasetimings related to the structure of the reference object (e.g.,particular phase timings that are expected to provide a best-focus imageof particular FS regions of the reference object 388) when the VFL lenssystem 300 is operating in a stable manner in a calibrated or referencestate.

FIG. 4 includes two related diagrams. A first embodiment of a referenceobject 488 is shown in a plan view in diagram 400A. The diagram 400B isa side view diagram, wherein the reference object 488 is shown tilted ata tilt angle TA, as imaged using the reference object opticsconfiguration 487, in a first implementation of focus state referencesubsystem 486 according to principles disclosed herein. The focus statereference subsystem 486 may be operated as, or substitute for, the focusstate reference subsystem 386 described with reference to FIG. 3 and itsoperation may be understood, in part, based on previous description.

As shown in the diagram 400A, the reference object 488 has a planar“pattern surface” that includes a contrast pattern CP arranged along atleast a portion of its periphery. It is generally desirable that thecontrast pattern CP include high contrast edges arranged at a highspatial frequency and to allow the determination of a quantitativecontrast or focus metric for relative small image regions (e.g., assmall as 9×9, or 5×5 pixels, or smaller in various implementations) in areference object image (e.g., as provided by the camera 360). As shownin FIG. 4, various focus state reference regions (FSRR) of the contrastpattern CP are arranged along the periphery (e.g., see representativeregions FSRR-10, FSRR-50 and FSRR-90) such that they are at differentfocus positions along the optical axis OAref, when the reference object488 is tilted at the tilt angle TA. It will be appreciated that thecontrast pattern CP may optionally be included in the areas RRopt, whichcould provide large FSRR's having the same focus position throughout,and which could provide a particularly high quality “focus curve” (thatis, a curve for a contrast or focus metric vs. imaging focus position),for determining a best-focus position and/or deviations compared to abest-focus position, as disclosed in the incorporated references.

FSRR's as referred to herein may be considered to be any region on areference object 488 that includes a contrast pattern CP at a referenceregion image location in reference object images, as may be known bydesign or calibration. Respective FSRRs are fixed at differentrespective reference region focus distances or reference focus positionsRFP relative to the reference object optics configuration 487. Thus,according to previously outlined principles and description, a cameraimage that includes a best-focus image of a particular FS referenceregion defines a system focus reference state associated with thatparticular FS reference region, and that defined system focus referencestate comprises at least one of a particular VFL optical power or aparticular effective focus position associated with that particular FSreference region.

In the example shown in FIG. 4, the contrast pattern CP may beunderstood to be distributed throughout a continuously distributed setof adjacent reference regions FSRR (and FSRR′) (which may beindividually designated FSRR-i, FSRR-i′, using a respective individualindex “i”). The reference regions may be defined to be as small asdesired, provided that a meaningful contrast or focus metric can bedetermined for them, so that it can be determined when they are imagedat a “best-focus” position. In the implementation shown in FIG. 4, thethree focus state reference regions FSRR-10, FSRR-50 and FSRR-90 will beunderstood to be representative of many more individual FSRRs in the setFSRR (or FSRR′). As shown in the diagram 400B, when the reference object488 is operationally arranged at the tilt angle TA, FSRR-10, FSRR-50 andFSRR-90 (and FSRR-10′, FSRR-50′, FSRR-90′) are arranged relative to thereference object optics configuration 487 at different reference focuspositions RFP-10, RFP-50 and RFP-90, respectively. It will beappreciated that other respective FSRR-i have other respective referencefocus positions RFP-i within a reference object focus position rangeRro.

In the focus state reference subsystem 486, the different respectiveFSRRs include at least one respective reference region focus distancelocated farther than Fref and at least one respective reference regionfocus distance located closer than Fref to the reference object opticsconfiguration 487. In one exemplary implementation, Fref may be at least30 millimeters, or at least 40 millimeters, or more. In one exemplaryembodiment, Fref may be 36 millimeters, and the tilt angle TA may beabout 25-40 degrees (e.g., 37 degrees), to provide a desired referenceobject focus range Rro. The dimensions of the reference object 488 maybe approximately 2.0×1.0 millimeters along the axes Yro and Yro,respectively. Any reference object disclosed herein may incorporate asomewhat diffusive or scattering surface with any imaged patternsurface, to allow more robust imaging, reduced alignment requirements,and reduced unwanted reflections. Such a focus state reference subsystem486 may be incorporated into an exemplary VFL lens system such as thosedescribed with reference to FIGS. 1-3, and imaged onto the periphery of1280×1024 pixel camera (e.g., the camera 360) to provide a referenceobject image usable according to principles disclosed herein.

When the focus state reference subsystem 486 is used in a system such asthat previously outlined with reference to FIG. 3 (e.g., in place of thefocus state reference subsystem 386), the reference object opticsconfiguration 487 may comprise a lens which transmits reference objectlight 389 that arises from the FS reference object 488 toward the beamsplitter 390. The partially reflecting beam splitter 390 directs thelight 389 of the reference object image along the imaging optical pathOPATH, which passes through the VFL lens 370 and emerges as light 389′which forms the image of the reference object 488 in the referenceobject image exposure that is produced by the camera 360 to supportfocus state monitoring, as described in greater detail below withreference to FIGS. 5, 6 and 7.

FIGS. 5A, 5B and 5C are diagrams representing three camera images thatinclude images of the reference object 488 of FIG. 4 in three differentfocus states. In particular, FIG. 5A represents a reference object imageROI-Ph-10 focused at the reference focus position RFP-10 where theFSRR-10 is best-focused; FIG. 5B represents a reference object imageROI-Ph-50 focused at the reference focus position RFP-50 where FSRR-50is best-focused; and FIG. 5C represents reference object image ROI-Ph-90focused at the reference focus position RFP-90 where the FSRR-90 isbest-focused. The reference focus positions RFP-10, RFP-50, RFP-90, areshown in FIG. 4. Different degrees of image focus or blur areschematically represented in FIGS. 5A, 5B and 5C by differentcross-hatch patterns. As shown, representative FSRR-10 (FSRR-10′) has animage location reference region image location (RRIL) RRIL-10(RRIL-10′), FSRR-50 (FSRR-50′) has an image location reference regionimage location RRIL-50 (RRIL-50′), and so on.

As previously indicated, the image focus location or position (e.g., atthe camera 360) is periodically altered by the periodic optical powervariation associated with the operation of the VFL lens 370. It will beappreciated that the point conjugate to the camera image plane, that is,the reference focus position RFP in the vicinity of the reference object488, is thus also periodically altered or swept due to the periodicoptical power variation associated with the operation of the VFL lens370. When different respective FSRRs are located at different respectivereference focus positions RFP, they will thus be best-focused inrespective images acquired at different respective times (designatedphase times) in relation to a phase or period of the periodic opticalpower variation of the VFL lens 370.

Accordingly, the reference object image ROI-Ph-10 in FIG. 5A is acquiredby an image exposure (e.g., a strobe timing) at a phase timingdesignated Ph-10 (according to a convention used herein) that provides acamera image wherein the FSRR-10 at RRIL-10 is best-focused at thereference focus position RFP-10. In this image, due to the tilt angle TAof the reference object 488 (as shown in FIG. 4) the focus of the FSRRsprogressively degrades as a function of distance away from FSRR-10 andRRIL-10, and is worst in the vicinity of FSRR-90 at RRIL-90, which is atthe far end of the reference object 488, and thus farthest from theimage focus position. The degree focus or blur for any FSRR in an imagemay be determined based on determining a contrast or focus metric forthat particular FSRR, according to known methods. FIGS. 5B and 5C may beunderstood by analogy to the previous description.

Briefly, the reference object image ROI-Ph-50 in FIG. 5B is acquired byan image exposure at a phase timing Ph-50 that provides a camera imagewherein the FSRR-50 at RRIL-50 is best-focused at the reference focusposition RFP-50. In this particular example, RFP-50 (shown in FIG. 4) isan example of a nominal focus position RFP-nom, which is designated tonominal focus position wherein the VFL optical power is zero. Thus,RFP-nom (e.g., RFP-50) may be in the middle of the focus range Rro, andmay correspond to a nominal focal length Fref of the reference objectoptics configuration 487 (e.g., a lens included therein). As RFP-50 iscentrally located along the reference object 488 and the focus positionrange Rro, the focus of the FSRRs progressively degrades in eachdirection away from FSRR-50 and RRIL-50, and is worst at the ends of thereference object 488. Briefly, the reference object image ROI-Ph-90 inFIG. 5C is acquired by an image exposure at a phase timing Ph-90 thatprovides a camera image wherein the FSRR-90 at RRIL-90 is best-focusedat the reference focus position RFP-90. The focus of the FSRRsprogressively degrades in the direction away from FSRR-90 and RRIL-90,and is worst in the vicinity of FSRR-10 at RRIL-10, at the far end ofthe reference object 488.

It should be appreciated that distributing the FSRRs along the peripheryof the reference object 488 allows them to be imaged such that theirrespective known RRILs in reference object images are located along oneor more edges of the camera image, and not in a central area of thecamera image. In some such implementations, the VFL lens system 300 maybe configured such that the workpiece image is located in the centralarea of the camera image. In some such implementations, a workpieceimage and a reference object image may be exposed simultaneously in thesame camera image, without using a separate timing or special wavelengthfiltering to prevent destruction of the workpiece image by the referenceobject image, or vice versa. While this configuration is particularlyadvantageous, it is not limiting. More generally, the VFL lens systemmay be configured such that the workpiece image is located in any firstpredetermined area of the camera image, and the respective known RRILsof the set of FSRRs are located in any second predetermined area of thecamera image that is different than the first predetermined area, or theworkpiece images and reference object images may be exposed at differenttimes, and significant advantages of a focus state reference subsystemaccording to this disclosure may still be provided.

It should be appreciated that for the reference object 488, wherein theFSRRs are continuously distributed throughout a focus position rangeRro, that any particular camera image may include a best-focused FSRR-i,if desired. As previously indicated, the VFL lens system 300 maycomprise a reference region focus analyzer 384 configured to identify abest-focus image of a particular FSRR in the camera image. In this case,the reference region focus analyzer 384 may analyze all the FSRRs in acamera image and determine the FSRR-i that has the best contrast orfocus metric. Importantly, that best-focused FSRR-i is directlyindicative of the focus state of the VFL lens system and its effectivefocus position EFP for that camera image, regardless of whether the VFLlens system drifts from its nominal or desired operating state. When thecamera image also includes a focused workpiece image, that EFP providesan accurate workpiece Z-height measurement regardless of whether the VFLlens system drifts from its nominal or desired operating state, asdescribed in greater detail below with reference to FIG. 6.

FIG. 6 is a chart 600 representing the relationships between variousfocus state features or parameters associated with various “bestfocused” focus state reference regions (FSRRs) of the reference object488 shown in FIG. 4, when imaged in various images such as thoserepresented in FIG. 5, for a VFL imaging system that includes the focusstate reference subsystem 486 described above, or the like. Thehorizontal axis includes positions that designate various FSRRs and/orRRILs that are best-focused in any particular image. It will beunderstood that the three explicitly designated focus state referenceregions FSRR-10, FSRR-50 and FSRR-90 (and/or their corresponding RRILs)are representative of many more individual FSRRs in the set FSRR, whichmay be represented along the horizontal axis.

A reference object calibration curve ROCC indicates a calibrated orknown relationship between each FSRR (or its corresponding RRIL) and thecorresponding reference focus position RFP shown on the vertical axis atthe right side of chart 600, where it is best-focused (e.g., asindicated by a focus metric and/or or focus curve determined by thereference region focus analyzer 384). Because the reference object 488includes a set of adjacent reference regions FSRR continuouslydistributed along a tilted plane as shown in FIG. 4, the referenceobject calibration curve ROCC is nominally a straight line, as shown. Inpractice, the reference object calibration curve ROCC may deviate from astraight line due to various design factors and/or imperfections, andmay be established by analysis or experiment, corresponding to anyparticular reference object 488, reference object optics configuration487, and VFL lens system 300.

Regarding the relationship between the three vertical axes in the chart600, the corresponding points on each vertical axis need not be linearlyrelated. For example, the optical power (OP) of the VFL lens isgenerally inversely related to its focal length, and therefore theoptical power axis may have a non-linear relationship to the referencefocus position axis and the effective focus position axis. Imperfectionsin any particular VFL lens system may also contribute to non-linearitiesor distortions between the various vertical axes. Nevertheless, the VFLlens is included as a “common mode” factor in the optical path OPATH forboth the effective focus position and the reference focus position, andall optical components are otherwise mounted in a fixed and stablerelationship to one another. Therefore, it may be considered that thethree vertical axes in the chart 600 are in a fixed and stablerelationship to one another. It will be understood that the effectivefocus position EFP in front of the objective lens (e.g., the lens 350)during an image exposure corresponds to the optical power of the VFLlens during that image exposure. Similarly, the reference focus positionRFP of the reference object optics configuration (e.g., theconfiguration 487) during an image exposure corresponds to the opticalpower of the VFL lens during that image exposure.

The relationship between the axes for any single state of the opticalpower provided by the VFL lens (e.g., the VFL lens 370) may bedetermined by analysis or experiment, and may thereafter be presumedstable (at least over the short term) in various implementations. Basedon the foregoing description, it will be understood that according tothe chart 600, a best-focused FSRR-i in a camera image may directlyindicate the focus state of the VFL lens system during that cameraimage, that focus state including the state of the optical power of theVFL lens 370 and the effective focus position EFP of the VFL lens system300. When that camera image also includes a focused workpiece image,that EFP provides an accurate workpiece Z-height measurement regardlessof whether the VFL lens system drifts from its nominal or desiredoperating state.

FIG. 7 is a chart 700 representing the relationships between variousfocus state features or parameters associated with a periodicallymodulated focus state of a VFL imaging system that includes a focusstate reference subsystem according to principles disclosed herein. Thehorizontal axis includes positions that designate various phase timingsthat maybe used to acquire a particular image. It will be understoodthat the explicitly designated phase timings are representative of otherphase timings relative to the periodic modulation of the VFL lens 370,which may be represented along the horizontal axis. The three verticalaxes are the same as those shown in FIG. 6, and are in a fixedrelationship to one another, as previously described.

A “reference level” modulation curve ModRS indicates a desired orreference state in which the VFL lens system is operating at a desiredor calibrated “reference state”, wherein the optical power of the VFLlens 370 has a “reference level” modulation amplitude of RangeRS. In thereference state corresponding to the reference level modulation curveModRS, particular phase timings are related to particular referencefocus positions RFP. For example, the phase timings PhRS50-1 andPhRS50-2 are related to RFP-50 at respective points P0 and P4 on thecurve ModRS. The FSRR-50 should be best focused in such images.Similarly, the phase timings PhRS90-1 and PhRS90-2 are related to RFP-90at respective points P2 and P3 on the curve ModRS. The FSRR-90 should bebest focused in such images, and so on. Because the vertical axes are ina fixed relationship, this means that a particular phase timing used tocapture a particular workpiece image is also indicative of the effectivefocus position of that workpiece image, provided that the VFL lenssystem 300 is operating in the reference state corresponding to thereference level modulation curve ModRS.

In some implementations, rather than indicating an effective focusposition EFP by analyzing all FSRRs in an image and determining thebest-focused FSRR-i, as outlined above with reference to FIGS. 4, 5 and6, it may be advantageous to maintain the reference state correspondingto the reference level modulation curve ModRS, and indicate an effectivefocus position EFP simply based on the phase timing used to acquire animage. The reference state may be maintained based on limited number ofreference object images that include best focus FSRRs at correspondingknown RRILs. Drifts from the reference state generally may be due todrifts in the resonant characteristics of a VFL lens 370 (e.g., theresonant amplitude or frequency of a TAG lens, or the like), forexample. If the resonant amplitude decreases to a level corresponding tothe modulation curve ModErr, for example, the phase timings PhRS90-1 andPhRS90-2 will no longer correspond to RFP-90. Rather, they willcorrespond to [(RFP-90)−(ΔFP90-1)] and [(RFP-90)−(ΔFP90-2)],respectively. Similarly, the phase timings PhRS10-1 and PhRS10-2 will nolonger correspond to RFP-10. Rather they will correspond to[(RFP-10)−(ΔFP10-1)] and [(RFP-10)−(ΔFP10-2)], respectively, and so on.It will be appreciated from the foregoing discussion that, wheneverdesired, the reference region focus analyzer 384 may be operated toanalyze a limited number of images including different FSRRs that havecorresponding known and stable FRPs and determine whether or notreference object images acquired at particular phase timings deviatefrom the reference state. The adjustment and/or warningcircuits/routines 385 of the lens controller 380 may then be operated toadjust the drive signal amplitude (and/or frequency) of the VFL lens370, to reestablish the reference state.

Alternatively, rather than maintaining the reference state, arelationship or transformation may be determined between the known(e.g., stored or calibrated) focus parameters of the reference state andthe focus parameters of a current operating state. For example, it willbe understood that it is possible to acquire reference object images ofthe FSRR-90 at various phase timings close to PhRS90-1 (or PhRS90-2)until a phase timing varying by ΔPh90-1 (or ΔPh90-2) produces a bestfocus image of FSRR-90 at RFP-90. Similar operations may be carried outin relation to other FSRRs, if desired. It will be appreciated that thereference level modulation curve ModRS (which may be previouslycharacterized and stored) can then be transformed to fit the resultscorresponding to the determined values for ΔPh90-1 (and/or ΔPh90-2), andthe like. The transformation can thereafter be used to determine acurrent relationship between the horizontal and vertical axes shown inthe chart 700 (e.g., to characterize the modulation curve ModErr), whichcan then be used to establish the effective focus position EFP forvarious images based on their phase timing, and so on.

According to the description immediately above, in contrast to thereference object 488, a reference object which includes a limited numberof FSRRs at a limited number of reference focus positions may be used toprovide a number of benefits such as maintaining a reference focusstate, or determining deviations from a reference focus state. One suchreference object is described below.

FIG. 8 includes two related diagrams, similar to those shown in FIG. 4,and including similar reference number designations. Similarly numbered(e.g., 8XX and 4XX) or named elements in FIG. 8 and FIG. 4 may providesimilar functions and may be understood by analogy, except as otherwiseindicated below.

A second embodiment of a reference object 888 is shown in a plan view indiagram 800A. The diagram 800B is a side view diagram, wherein thereference object 888 is shown as imaged using the reference objectoptics configuration 887, in a second implementation of focus statereference subsystem 886, according to principles disclosed herein. Thefocus state reference subsystem 886 may be operated as, or substitutefor, the focus state reference subsystem 386 described with reference toFIG. 3 and its operation may be understood, in part, based on theprevious description.

As shown in the diagrams 800A and 800B, the reference object 888 hasthree planar “pattern surfaces”, Layer0, Layer2, and Layer1 that includea contrast pattern CP arranged in various focus state reference regions(FSRR) at their periphery. Layer0, Layer2, and Layer1 are at differentfocus positions along the optical axis OAref within (and defining) thereference object focus range Rro. It will be understood that the FSRRsmay optionally be included in the more or fewer areas, or may bedifferently sized, if desired. It will be understood that FSRRs on thesame layer nominally correspond to the same RFP (e.g., FSRR-21 andFSRR-22 are both located at RFP-20, and so on), and are thereforeredundant to some degree. However, such redundancy may help overcomevarious alignment or assembly errors, or provide other benefits, in someembodiments. It will be understood that in other implementations, twolayers or more than three layers may be used, if desired.

Similar to the focus state reference subsystem 486, in the focus statereference subsystem 886 the reference object optics configuration 887may have a focal distance Fref (e.g., of a lens included therein) alongits optical axis, and the different respective FSRRs include at leastone respective reference region focus distance located farther than Frefand at least one respective reference region focus distance locatedcloser than Fref to the reference object optics configuration 887. Inone exemplary implementation, Fref may be at least 30 millimeters, or atleast 40 millimeters, or more. In one exemplary embodiment, Fref may be36 millimeters, and the thickness between layers may be approximately0.32 millimeters, and the dimensions of the reference object 888 may beapproximately 1.0×1.25 millimeters along the axes Xro and Yro,respectively. Such a focus state reference subsystem 886 may beincorporated into an exemplary VFL lens system such as those describedwith reference to FIGS. 1-3, and imaged onto the periphery of 1280×1024pixel camera (e.g., the camera 360) to provide a reference object imageusable according to principles disclosed herein.

FIGS. 9A, 9B and 9C are diagrams representing three camera images thatinclude images of the reference object 888 of FIG. 8 in three differentfocus states. In particular, FIG. 9A represents a reference object imageROI-Ph-10 focused at the reference focus position RFP-10 where FSRR-11,-11′, -12, and -12′ on Layer1 are best-focused; FIG. 9B represents areference object image ROI-Ph-00 focused at the reference focus positionRFP-00 where FSRR-01, -01′, -02, -02′, -03 and -03′ on Layer0 arebest-focused; and FIG. 9C represents reference object image ROI-Ph-20focused at the reference focus position RFP-20 where FSRR-21, -21′, -22,and -22′ on Layer2 are best-focused. The reference focus positionsRFP-10, RFP-00, RFP-20, are shown in FIG. 8.

It will be appreciated that the contrast pattern(s) CP shown in FIGS. 8and 9 are exemplary only and not limiting. For example, the contrastpatterns on various layers may be longer along the direction Yro (e.g.,as illustrated in FIG. 4), but offset to one another along the directionXro, so that they do not interfere and can be separated in a cameraimage. Other possible forms of the contrast pattern CP will be apparentbased on the foregoing disclosure.

It will be understood the limited number of FSRRs and correspondingknown reference focus positions RFP of the focus state referencesubsystem 886 described above with reference to FIGS. 8 and 9 may beused to provide a number of benefits such as maintaining a referencefocus state, or determining deviations from a reference focus state, aspreviously outlined above with reference to FIG. 7.

In various implementations outlined above, image data acquired by thecamera 360 as the basis for monitoring or determining the focus state ofthe VFL system and may in some instances be displayed in a userinterface of the imaging system 300 (e.g., see FIG. 2, display devices136), or alternatively may not be outputted by the imaging system 300(e.g., utilized primarily internally for determining effective focusdistances and/or making adjustments, etc.).

In some implementations, a focus state reference subsystem configuredaccording to principles outlined above may be provided as an “accessory”for use with a variable focal length (VFL) lens system that includes aVFL lens system frame or housing comprising an accessory optical port.In such an implementation, in addition to the elements outlined above,the focus state reference subsystem may comprise a reference subsystemframe or housing comprising a reference subsystem optical portconfigured to mount fixedly to the accessory optical port on the VFLlens system. In such implementations, the reference object opticsconfiguration is configured such that when the reference subsystemoptical port is mounted fixedly to the accessory optical port on the VFLlens system, the reference object optics configuration is arranged toinput reference object light arising from the FS reference object duringa reference object image exposure, and transmit the reference objectlight along at least a portion of the imaging optical path to passthrough the VFL lens and to the camera during the reference object imageexposure, to provide a reference object image in a corresponding cameraimage, for use as previously outlined and as described in greater detailbelow.

In a variation of the implementation described with reference to thetilted reference object 488, a reference object may more generallycomprise at least one pattern surface which is curved, at least part ofwhich is not perpendicular to an optical axis of the reference objectoptics configuration. Different portions of the at least one curvedpattern surface are fixed at different respective focus distancesrelative to the reference object optics configuration, and the set ofFSRRs are arranged on the different portions of the at least one curvedpattern surface.

In some implementations, the features and principles outlined above maybe used to establish a calibrated state for a VFL lens system. Thecalibrated state may comprise at least one member of the set of FSRRsexhibiting a particular calibrated focus characteristic value in areference object image exposed using a corresponding particular knownphase timing. The reference region focus analyzer 384 may be configuredto determine the focus characteristic value for members of the set ofFSRRs in reference object images exposed using corresponding particularknown phase timings. In some implementations, the VFL lens controller380 may be configured to automatically or semi-automatically operate thereference region focus analyzer 384 to adjust the drive signal of theVFL lens 370 such that the determined focus characteristic value for atleast one member of the set of FSRRs in at least one reference objectimage exposed using a corresponding particular known phase timingmatches the particular calibrated focus characteristic value for that atleast one member of the set of FSRRs at that corresponding particularknown phase timing. In some implementations, the VFL lens system may beconfigured to automatically or semi-automatically operate the referenceregion focus analyzer 384 and provide a warning indicator when thedetermined focus characteristic value for at least one member of the setof FSRRs in at least one reference object image exposed using acorresponding particular known phase timing does not match theparticular calibrated focus characteristic value for that at least onemember of the set of FSRRs at that corresponding particular known phasetiming.

In some implementations that include a calibrated state as outlinedabove, the determined focus characteristic value for a member of the setof FSRRs may comprise the value of a quantitative contrast metric thatis based on reference object image data corresponding to the known RRILof that member of the set of FSRRs. In some such implementations, thereference region focus analyzer 384 may comprise a set of operationsimplemented by a software routine in a remote computer, wherein theremote computer is configured to receive reference object image datafrom the VFL system and perform the set of operations to determine thevalue of the quantitative contrast metric based on the reference objectimage data corresponding to the known RRIL of that member of the set ofFSRRs.

While preferred implementations of the present disclosure have beenillustrated and described, numerous variations in the illustrated anddescribed arrangements of features and sequences of operations will beapparent to one skilled in the art based on this disclosure. Variousalternative forms may be used to implement the principles disclosedherein. For example, a multi-layer reference object similar to thatdescribed with reference to FIGS. 8 and 9, but including only twolayers, may be used in various implementations. In addition, the variousimplementations described above can be combined to provide furtherimplementations. For example, a tilted reference object may beimplemented using an adjustable tilting mechanism, so that the referenceobject focus position range Rro covered by the tilted reference objectmay be adjusted to correspond to a particular objective lens, or toincrease the focus distance selectivity or resolution associated withvarious focus state reference region locations, or the like. As anotherexample, a multi-layer reference object may be tilted to provide acombination of the benefits associated with each of a multi-layerreference object and a tilted reference object. All of the U.S. patentsand U.S. patent applications referred to in this specification areincorporated herein by reference, in their entirety. Aspects of theimplementations can be modified, if necessary to employ concepts of thevarious patents and applications to provide yet further implementations.

These and other changes can be made to the implementations in light ofthe above-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificimplementations disclosed in the specification and the claims, butshould be construed to include all possible implementations along withthe full scope of equivalents to which such claims are entitled.

The invention claimed is:
 1. A variable focal length (VFL) lens system, comprising: a VFL lens; a VFL lens controller that controls a drive signal of the VFL lens to periodically modulate optical power of the VFL lens over a range of optical powers that occur at respective phase timings within the periodic modulation; a camera that receives light transmitted along an imaging optical path through the VFL lens during an image exposure and provides a corresponding camera image; an objective lens that inputs workpiece light arising from a workpiece during a workpiece image exposure and transmits the workpiece light along the imaging optical path through the VFL lens and to the camera during the workpiece image exposure, to provide a workpiece image in a corresponding camera image, wherein an effective focus position in front of the objective lens during a workpiece image exposure corresponds to the optical power of the VFL lens during that workpiece image exposure; and an exposure time controller configured to control an image exposure timing used for a camera image, wherein: the VFL lens system further comprises a focus state reference subsystem comprising at least a focus state (FS) reference object and a reference object optics configuration; the reference object optics configuration is arranged to input reference object light arising from the FS reference object during a reference object image exposure, and transmit the reference object light along at least a portion of the imaging optical path to pass through the VFL lens and to the camera during the reference object image exposure, to provide a reference object image in a corresponding camera image; and the FS reference object comprises a set of focus state (FS) reference regions that include a contrast pattern and that have respective known reference region image locations in the reference object images and that are fixed at different respective reference region focus positions relative to the reference object optics configuration, wherein a camera image that includes a best-focus image of a particular FS reference region defines a system focus reference state associated with that particular FS reference region, and that defined system focus reference state comprises at least one of a particular VFL optical power or a particular effective focus position associated with that particular FS reference region.
 2. The VFL lens system of claim 1, wherein the FS reference object comprises a plurality of planar pattern surfaces fixed at different respective focus distances relative to the reference object optics configuration along an optical axis of the reference object optics configuration, and the set of FS reference regions are arranged on the plurality of planar pattern surfaces.
 3. The VFL lens system of claim 1, wherein the FS reference object comprises at least one pattern surface, at least part of which is not perpendicular to an optical axis of the reference object optics configuration, such that different portions of the at least one pattern surface are fixed at different respective focus distances relative to the reference object optics configuration, and the set of FS reference regions are arranged on the different portions of the at least one pattern surface.
 4. The VFL lens system of claim 3, wherein the at least one pattern surface comprises a planar pattern surface that is not normal to the optical axis of the reference object optics configuration, and the set of FS reference regions comprise different portions of a contrast pattern that extends along the planar pattern surface that is not normal to the optical axis of the reference object optics configuration.
 5. The VFL lens system of claim 1, wherein the reference object optics configuration comprises a reference object imaging lens having a focal distance Fref along an optical axis of the reference object optics configuration, and the different respective reference region focus positions include at least one respective reference region focus position located farther than Fref from the reference object imaging lens and at least one respective reference region focus position located closer than Fref to the reference object imaging lens.
 6. The VFL lens system of claim 5, wherein Fref is at least 30 millimeters.
 7. The VFL lens system of claim 1, wherein the set of FS reference regions are configured on the FS reference object such that their respective known reference region image locations in the reference object images are located along one or more edges of the camera image, and not in a central area of the camera image.
 8. The VFL lens system of claim 7, wherein the VFL lens system is configured such that the workpiece image is located in the central area of the camera image.
 9. The VFL lens system of claim 1, wherein the VFL lens system is configured such that the workpiece image is located in a first predetermined area of the camera image, and the respective known reference region image locations of the set of FS reference regions are located in a second predetermined area of the camera image that is different than the first predetermined area, and the workpiece image and the reference object image are exposed simultaneously in the same camera image.
 10. The VFL lens system of claim 9, wherein the VFL lens system comprises a reference region focus analyzer configured to identify the best-focus image of the particular FS reference region in the camera image, and identify the particular effective focus position associated with that particular FS reference region as an effective focus position of the workpiece image in the same camera image.
 11. The VFL lens system of claim 1, wherein: a calibrated state of the VFL lens system comprises at least one member of the set of FS reference regions exhibiting a particular calibrated focus characteristic value in a reference object image exposed using a corresponding particular known phase timing; the VFL lens system comprises a reference region focus analyzer configured to determine the focus characteristic value for members of the set of FS reference regions in reference object images exposed using corresponding particular known phase timings, and the VFL lens controller is configured to automatically or semi-automatically operate the reference region focus analyzer and adjust the drive signal of the VFL lens such that the determined focus characteristic value for at least one member of the set of FS reference regions in at least one reference object image exposed using a corresponding particular known phase timing matches the particular calibrated focus characteristic value for that at least one member of the set of FS reference regions at that corresponding particular known phase timing.
 12. The VFL lens system of claim 11, wherein the determined focus characteristic value for a member of the set of FS reference regions comprises a value of a quantitative contrast metric that is based on reference object image data corresponding to the known reference region image location of that member of the set of FS reference regions.
 13. The VFL lens system of claim 12, wherein the reference region focus analyzer comprises a set of operations implemented by a software routine in a remote computer, the remote computer configured to receive the reference object image data from the VFL lens system and perform the set of operations to determine the value of the quantitative contrast metric based on the reference object image data corresponding to the known reference region image location of that member of the set of FS reference regions.
 14. The VFL lens system of claim 1, wherein; the VFL lens system comprises a workpiece strobe light source configured to provide illumination to the workpiece during the workpiece image exposure; and the exposure time controller is configured to control a timing of the workpiece image exposure by controlling a strobe timing of the workpiece strobe light source.
 15. The VFL lens system of claim 14, wherein; the VFL lens system comprises a reference object strobe light source configured to provide illumination to the FS reference object during the reference object image exposure; and the exposure time controller is configured to control a timing of the reference object image exposure by controlling a strobe timing of the reference object strobe light source.
 16. The VFL lens system of claim 15, wherein the workpiece strobe light source and the reference object strobe light source are the same light source, the timing of the workpiece image exposure and the timing of the reference object image exposure are simultaneous, and the workpiece image and the reference object image are included in the same camera image.
 17. The VFL lens system of claim 15, wherein the workpiece strobe light source and the reference object strobe light source are different light sources, the timing of the workpiece image exposure and the timing of the reference object image exposure are different, and the workpiece image and the reference object image are included in different camera images.
 18. The VFL lens system of claim 1, wherein: a calibrated state of the VFL lens system comprises at least one member of the set of FS reference regions exhibiting a particular calibrated focus characteristic value in a reference object image exposed using a corresponding particular known phase timing; the VFL lens system comprises a reference region focus analyzer configured to determine the focus characteristic value for at least one member of the set of FS reference regions in reference object images exposed using corresponding particular known phase timings, and the VFL lens system is configured to automatically or semi-automatically operate the reference region focus analyzer and provide a warning indicator when the determined focus characteristic value for at least one member of the set of FS reference regions in at least one reference object image exposed using a corresponding particular known phase timing does not match the particular calibrated focus characteristic value for that at least one member of the set of FS reference regions at that corresponding particular known phase timing.
 19. A focus state reference subsystem for use in a variable focal length (VFL) lens system, wherein: the VFL lens system comprises: a VFL lens; a VFL lens controller that controls a drive signal of the VFL lens to periodically modulate optical power of the VFL lens over a range of optical powers that occur at respective phase timings within the periodic modulation; a camera that receives light transmitted along an imaging optical path through the VFL lens during an image exposure and provides a corresponding camera image; an objective lens that inputs workpiece light arising from a workpiece during a workpiece image exposure and transmits the workpiece light along the imaging optical path through the VFL lens and to the camera during the workpiece image exposure, to provide a workpiece image in a corresponding camera image, wherein an effective focus position in front of the objective lens during a workpiece image exposure corresponds to the optical power of the VFL lens during that workpiece image exposure; an exposure time controller configured to control an image exposure timing used for a camera image; and a VFL lens system frame or housing comprising an accessory optical port, and the focus state reference subsystem comprises at least a focus state (FS) reference object, a reference object optics configuration, and a reference subsystem frame or housing comprising a reference subsystem optical port, wherein; the reference subsystem optical port is configured to mount fixedly to the accessory optical port on the VFL lens system; the reference object optics configuration is configured such that when the reference subsystem optical port is mounted fixedly to the accessory optical port on the VFL lens system, the reference object optics configuration is arranged to input reference object light arising from the FS reference object during a reference object image exposure, and transmit the reference object light along at least a portion of the imaging optical path to pass through the VFL lens and to the camera during the reference object image exposure, to provide a reference object image in a corresponding camera image; and the FS reference object comprises a set of focus state (FS) reference regions that include a contrast pattern and that have respective known reference region image locations in the reference object images and that are fixed at different respective reference region focus positions relative to the reference object optics configuration, wherein a camera image that includes a best-focus image of a particular FS reference region defines a system focus reference state associated with that particular FS reference region, and that system focus reference state comprises at least one of a particular VFL optical power or a particular effective focus position associated with that particular FS reference region. 